Optical concentrating systems, such as solar collectors, concentrate light toward a focus of the optical system. In general, there are two categories of concentrators. Line concentrators concentrate incident light in one dimension so that the focus is a line. Point concentrators concentrate incident light in two dimensions so that the focus is a point.
Concentrators may include one or more optical components to concentrate incident light. Some systems have a single concentrating optical component, referred to as the primary optic, that concentrates rays directly onto the desired target (which may be a device such as a photovoltaic cell) after being collected and focused by the optic. More complex concentrators may include both a primary optic and additional optics to provide further collection or concentration abilities or improve beam uniformity at the target.
A primary optic for an optical concentrator typically includes either a refractive component or a reflective component. The most common refractive component employed is a Fresnel lens, as in O'Neill, U.S. Pat. No. 4,069,812, while a common reflective component is a parabolic reflector. With respect to refractive components, Fresnel lenses are usually preferred over standard lenses, because Fresnel lenses are thinner for a given aperture. As such, Fresnel lenses allow large collecting apertures without requiring as much lens material as does a standard lens. The system aperture for these concentrators is defined by the aperture of the Fresnel lens. FIG. 2 illustrates a typical Fresnel lens concentrator having optical axis 24, showing a Fresnel lens 18 bending light rays 20 towards a desired focus 16.
Large, high quality Fresnel lenses as conventionally used, however, can be prohibitively expensive for medium scale applications such as commercial rooftop systems. In addition, surface discontinuities present on Fresnel lenses tend to make them lossy (i.e., inasmuch as some of the light that is desirably focused may instead be absorbed and/or directed away from the focus) compared to standard lenses or reflective solutions. Another disadvantage of the Fresnel concentrator as conventionally used is that it is not suitable by itself for certain articulating concentrators that require self-powering. Such devices require a means to generate power when the optical axis of the concentrator is not aligned with the sun thereby relying on diffuse radiation from the sky. Unfortunately, a conventional Fresnel concentrator provides negligible paths for diffuse radiation to strike a solar cell located at the focus of the lens and therefore is usually unable to generate sufficient power to articulate itself when not aligned with the sun.
Reflective primaries are known to include compound parabolic concentrators (CPCs) as per Winston, U.S. Pat. No. 4,003,638 well as various types of parabolic or nearly parabolic troughs and dishes. CPCs provide good beam uniformity at the focal plane. Troughs and dishes are the two main types of CPC's. Troughs and dishes may have a bottom focus wherein the optical target, for example a solar cell, is facing up. Troughs and dishes with a bottom focus advantageously may collect and concentrate diffuse light even when the reflector is not directly aimed at the source(s) of the diffuse light. This makes them suitable for collecting diffuse light used for self-power. Troughs and dishes also may have an inverted focus wherein the optical target, for example a solar cell, is facing down, often suspended above the reflector.
However, because high concentration ratios tend to require a CPC with a large height/width ratio, the packing density for multiple articulating concentrators including CPC's can be limited. For example, FIG. 3 of the present application schematically illustrates a typical bottom focus CPC 28 with a geometric concentration of 10× in one dimension. Incident rays 30 are concentrated onto focal plane 26 with a normalized width of 1. The normalized height of the CPC 28 is 17.8, resulting in a height/width ratio of 1.78.
This relatively high height/width ratio factor makes conventional CPC's, by themselves, poorly suited for multiple articulating concentrator systems such as those described in Assignee's U.S. Provisional Patent Application No. 60/691,319, filed Jun. 16, 2005, in the names of Hines et al., titled PLANAR CONCENTRATING PHOTOVOLTAIC SOLAR PANEL WITH INDIVIDUALLY ARTICULATING CONCENTRATOR ELEMENTS and Assignee's U.S. Provisional Patent Application No. 60/759,778, filed Jan. 17, 2006, in the name of Hines, titled CONCENTRATING SOLAR PANEL AND RELATED SYSTEMS AND METHODS, which applications are incorporated herein by reference in their respective entireties for all purposes,. Such articulating concentrator systems desirably utilize a low overall height for the optical component, so that the concentrators can articulate freely.
As another drawback, parabolic troughs and dishes have aperture regions that are, in practice, unusable for concentrating. This is true for troughs and dishes that have either a bottom focus or an inverted focus. Portions of the apertures of these optical elements are unusable because both the bottom and inverted focusing configurations can be affected by angle of incidence limits at the target focal plane. For example, according to Snell's law, rays striking the target at greater than a certain angle are reflected off the surface and are not absorbed.
FIG. 4 schematically illustrates this issue for a bottom focus reflector 34. Incident rays 36, 38, 40 are concentrated by reflector 34 onto focal plane 32. The angle of incidence with respect to the focal plane 32 of the concentrated rays is greater for rays closer to the optical axis (not shown), which extends through the middle of the reflector 34. Rays 36 and 38 impinge on focal plane 32 at angles less than the acceptance angle of the focal plane 32 and are absorbed. Ray 40 impinges on focal plane 32 at an angle greater than the acceptance angle of the focal plane and is reflected back out of the reflector 34 as ray 42. The same effect would be seen if ray 42 would be incident ray and ray 40 would be the rejected ray. The regions 44 and 46 associated with non-absorbed rays 44 and 42 define the portion of the reflector's 34 aperture that is not usable for concentrating. In practical effect, the effective aperture of the system is reduced.
Inverted focus reflectors suffer from a similar effect except that the aperture penalty occurs near the periphery of the reflector. As schematically illustrated in FIG. 5, incident rays 52, 56, 60 are concentrated by reflector 50 onto focal plane 48. In contrast to the situation with a bottom focus reflector, the angle of incidence with respect to the focal plane 48 of the reflector 50 increases as rays strike reflector 50 further away from the optical axis (not shown), which extends through the middle of the reflector. Rays 52 and 60 impinge on focal plane 48 at angles less than the acceptance angle of the focal plane 48 and are absorbed. Ray 56 impinges on focal plane 48 at an angle greater than the acceptance angle of the focal plane 48 and is reflected back out of the reflector 50 as ray 58. The same effect is seen if ray 58 would be the incident ray and ray 56 would be the rejected ray. The regions 54 and 62 of non-absorbed rays 54 and 62 define the portion of the reflector's aperture that is not usable for concentrating. In practical effect, this limits the width of the reflector's aperture. In addition, as is typical of inverted focus configurations, reflector 50 suffers from self-shadowing such that rays nearest to the optical axis in region 64 are blocked by the target at the focal plane 48 itself, further reducing the light-collecting efficiency of the system.
In addition, articulating concentrator systems desirably would include a means to power the articulating components, preferably using power generated by the device itself. Conventional optical designs can present challenges for photovoltaic devices that would like to use self-powered articulation to aim light concentrating components at the source of incident light, e.g., the sun. It is important that self-powered designs be able to capture and concentrate diffuse light to provide power when the light concentrating components are not aimed properly. Such devices may use bottom focus reflectors in order to provide sufficient optical paths for diffuse radiation to strike a solar cell located at the focal plane. However, as implemented conventionally, this design choice occurs at the expense of the aforementioned limitations of the bottom focus reflector. Devices that instead use inverted focus reflectors, on the other hand, generally provide only very limited optical paths for diffuse radiation to reach the target, as the target, e.g., a solar cell, is facing away from diffuse radiative sources. Also, the reflected field of view in the primary mirror tends to be very narrow. Consequently, inverted focus reflectors tend to collect little diffuse light. These conventional bottom focus and inverted focus reflectors are therefore not well-suited to self-powered systems.
A third type of concentrating primary, a reflective lens as described in Vasylyev, U.S. Pat. No. 6,971,756, includes reflective elements in the form of concentric rings or parallel slats arranged so that incident rays are focused like a lens. These primaries can provide large concentration ratios and may overcome angle of incidence issues present with parabolic troughs and dishes. However, these generally include multiple, precision aligned surfaces that may be cost-prohibitive for medium-scale applications. Additionally, in the case of a long parallel slat form, additional support structure may be required that would tend to create undesirable optical obscurations. Further, in a manner that is analogous to the limitations of a refractive Fresnel lens discussed above, such a design has a limited ability to collect and focus diffuse light to provide for self-powering.
Some attempts have been made in the prior art to improve upon these solutions by combining multiple optical elements into a single concentrator, e.g., as described by Habraken in U.S. Pat. Pub. No. 2004/0134531 and by Cobert in U.S. Pat. Pub. No. 2005/0067008. However, both of these approaches place the multiple optical elements in series, so that light is redirected by multiple elements before reaching the focus. The significant disadvantage of these approaches is that they incur the expense and optical losses of two separate, full-aperture optical elements.